Membrane electrode assembly with improved electrode

ABSTRACT

A membrane electrode assembly comprises a polymer electrolyte interposed between an anode electrode and a cathode electrode, the anode electrode comprising an anode catalyst layer adjacent at least a portion of a first major surface of the polymer electrolyte, the cathode electrode comprising a cathode catalyst layer adjacent at least a portion of a second major surface of the polymer electrolyte; at least one of the anode and cathode catalyst layers comprising: a first catalyst composition comprising a noble metal; and a second composition comprising a metal oxide; wherein the second composition has been treated with a fluoro-phosphonic acid compound.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a membrane electrode assembly with animproved electrode for use in PEM fuel cells, and to catalyst-coatedmembranes and fuel cells comprising the improved electrode.

Description of the Related Art

Fuel cell systems are currently being developed for use as powersupplies in numerous applications, such as automobiles and stationarypower plants. Such systems offer promise of delivering powereconomically and with environmental and other benefits. To becommercially viable, however, fuel cell systems should exhibit adequatereliability in operation, even when the fuel cells are subjected toconditions outside their preferred operating ranges.

Fuel cells convert reactants, namely, fuel and oxidant, to generateelectric power and reaction products. Polymer electrolyte membrane fuelcells (“PEM fuel cell”) employ a membrane electrode assembly (“MEA”),which comprises a polymer electrolyte or ion-exchange membrane disposedbetween the two electrodes, namely a cathode and an anode. A catalysttypically induces the desired electrochemical reactions at theelectrodes. Separator plates, or flow field plates for directing thereactants across one surface of each electrode substrate, are disposedon each side of the MEA.

In operation, the output voltage of an individual fuel cell under loadis generally below one volt. Therefore, in order to provide greateroutput voltage, multiple cells are usually stacked together and areconnected in series to create a higher voltage fuel cell stack. (Endplate assemblies are placed at each end of the stack to hold the stacktogether and to compress the stack components together. Compressiveforce provides sealing and adequate electrical contact between variousstack components.) Fuel cell stacks can then be further connected inseries and/or parallel combinations to form larger arrays for deliveringhigher voltages and/or currents.

In practice, fuel cells need to be robust to varying operatingconditions, especially in applications that impose numerous on-offcycles and/or require dynamic, load-following power output, such asautomotive applications. For example, fuel cell anode catalysts are alsopreferably tolerant to cell voltage reversals and carbon monoxidepoisoning; carbon-supported catalysts are also preferably resistant tocorrosion during start up and shutdown procedures.

PEM fuel cells typically employ noble metal catalysts, and it is wellknown that such catalysts, particularly platinum, are very sensitive tocarbon monoxide poisoning. This is a particular concern for the anodecatalyst of fuel cells operating on reformate, but it also a concern forfuel cells operating on hydrogen, as carbon monoxide (CO) is sometimespresent in the hydrogen supply as a fuel contaminant. As described by,e.g., Niedrach et al. in Electrochemical Technology, Vol. 5, 1967, p.318, the use of a bimetallic anode catalyst comprisingplatinum/ruthenium, rather than monometallic platinum, shows a reductionin the poisoning effect of the CO at typical PEM fuel cell operatingtemperatures. Hence, Pt—Ru catalysts are typically employed as PEM fuelcell anode catalysts.

Ruthenium-based fuel cell catalysts are also useful for mitigatingvoltage reversals. Voltage reversals occur when a fuel cell in a seriesstack cannot generate sufficient current to keep up with the rest of thecells in the series stack. Several conditions can lead to voltagereversal in a PEM fuel cell, for example, including insufficientoxidant, insufficient fuel, and certain problems with cell components orconstruction. Reversal generally occurs when one or more cellsexperience a more extreme level of one of these conditions compared toother cells in the stack. While each of these conditions can result innegative fuel cell voltages, the mechanisms and consequences of such areversal may differ depending on which condition caused the reversal.Groups of cells within a stack can also undergo voltage reversal andeven entire stacks can be driven into voltage reversal by other stacksin an array. Aside from the loss of power associated with one or morecells going into voltage reversal, this situation poses reliabilityconcerns. Undesirable electrochemical reactions may occur, which maydetrimentally affect fuel cell components. Component degradation reducesthe reliability and performance of the affected fuel cell, and in turn,its associated stack and array.

As described in U.S. Pat. No. 6,936,370, fuel cells can also be mademore tolerant to cell reversal by promoting water electrolysis overanode component oxidation at the anode. This can be accomplished byincorporating an additional catalyst composition at the anode to promotethe water electrolysis reaction. As a result, more of the current forcedthrough the fuel cell during voltage reversal can be consumed in theelectrolysis of water rather than the oxidation of anode components.Among the catalyst compositions disclosed were Pt—Ru alloys, RuO₂ andother metal oxide mixtures and/or solid solutions including Ru.

U.S. Patent Application No. 2004/0013935 also describes an approach toimproving cell voltage reversal tolerance by using anodes employing botha higher catalyst loading (at least 60 wt % catalyst) on an optionalcorrosion-resistant support, and incorporating certain unsupportedcatalyst compositions to promote the water electrolysis reaction.Disclosed preferred compositions include oxides characterized by thechemical formulae RuOx and IrOx, where x is greater than 1 andparticularly about 2, and wherein the atomic ratio of Ru to Ir isgreater than about 70:30, and particularly about 90:10.

However, ruthenium has been shown to be unstable under certain fuel celloperating conditions. For example, Piela et al. (J. Electrochem. Soc.,151 (12), A2053-A2059 (2004)), describe ruthenium crossover from Pt—Rublack catalyst and redeposition at the Pt cathode catalyst in directmethanol fuel cells (DMFC) and hydrogen/air fuel cells under abnormalconditions, such as cell reversal resulting in very high anodepotentials (and under normal DMFC operating conditions). Piela et al.theorized that the Pt—Ru alloy should likely remain stable under DMFCoperating conditions, and that the source of the ruthenium contaminationwas neutral hydrous RuO₂. Taniguchi et al. (J. Power Sources, 130, 42-49(2004)) observed ruthenium dissolution from a carbon supported Pt—Ruanode catalyst as a result of high anode potentials experienced by thefuel cell under cell reversal conditions.

It has also been shown that Pt—Ru catalysts are prone to rutheniumdissolution at higher relative humidity operation and cathode carboncorrosion. For example, P. He et al. (ECS Transactions, 33 (1) 1273-1279(2010)) found that relative humidity (RH) significantly impacted thedegree of ruthenium dissolution and crossover, which subsequentlyaffected the cell performance and CO tolerance. Lower operating RHduring testing resulted in less ruthenium contamination on the cathodeand lower performance losses. In addition, T. Cheng et al. (Journal ofThe Electrochemical Society, 157 (5) B714-B718 (2010)) investigatedanode catalysts with different elemental compositions to cause variousdegrees of ruthenium crossover. It was found that after anodeaccelerated stress test cycles, ruthenium crossover and subsequentdeposition on the cathode occurred, which resulted in significant fuelcell performance loss.

To mitigate ruthenium dissolution of Pt—Ru-based catalysts, U.S. Pat.No. 7,608,358 discloses an electrode assembly for a fuel cell comprisingan anode catalyst layer, the anode catalyst layer comprising a firstcatalyst composition comprising a noble metal, other than ruthenium, ona corrosion resistant support material; a second catalyst compositioncomprising a single-phase solid solution of a metal oxide containingruthenium; and a hydrophobic binder; wherein a through-planeconcentration of an ionomer in the catalyst layer decreases as afunction of distance from the electrolyte. '358 discloses that MEAS andfuel cells with such an electrode assembly demonstrated markedlyimproved cell reversal tolerance and performance in start/stop cyclingtests, while retaining baseline performance and performance in thepresence of CO. '358 also discloses the mechanisms for rutheniumcrossover from the anode and the cathode, particularly due to start/stopcycling.

However, ruthenium oxide-based catalysts, such as those described in'358, are still be prone to ruthenium dissolution. As a result, thereexists a need for membrane electrode assemblies and fuel cells that aremore robust to operating conditions that impose numerous on-off cyclesand/or require dynamic, load-following power output; are tolerant tocell voltage reversals; and resistant to carbon monoxide poisoning andcorrosion during start up and shutdown procedures. The present inventionaddresses this need and provides associated benefits.

BRIEF SUMMARY OF THE INVENTION

In brief, a membrane electrode assembly comprises a polymer electrolyteinterposed between an anode electrode and a cathode electrode, the anodeelectrode comprising an anode catalyst layer adjacent at least a portionof a first major surface of the polymer electrolyte, the cathodeelectrode comprising a cathode catalyst layer adjacent at least aportion of a second major surface of the polymer electrolyte; at leastone of the anode and cathode catalyst layers comprising: a firstcatalyst composition comprising a noble metal; and a second compositioncomprising a metal oxide; wherein the second composition has beentreated with a fluoro-phosphonic acid compound. In further embodiments,the metal oxide of the second composition is selected from the groupconsisting of ruthenium oxide, iridium oxide, ruthenium iridium oxide,titanium oxide, cerium oxide, and their mixtures, solid solutions andcomposites thereof.

In specific embodiments, the fluoro-phosphonic acid compound is afluoroalkyl-phosphonic acid compound. In further embodiments, thefluoro-phosphonic acid compound is 2-perfluorohexyl ethyl phosphonicacid. In other embodiments, the fluoro-phosphonic acid compound is(1H,1H,2H,2H-heptadecafluorodec-1-yl) phosphonic acid.

In further embodiments, the first catalyst composition is in a firstdiscrete layer and the second composition is in a second discrete layerin the at least one of the anode and cathode catalyst layers.

In another embodiment, a method of making a membrane electrode assembly,the membrane electrode assembly comprising a polymer electrolyteinterposed between an anode electrode and a cathode electrode, the anodeelectrode comprising an anode catalyst layer adjacent at least a portionof a first major surface of the polymer electrolyte, the cathodeelectrode comprising a cathode catalyst layer adjacent at least aportion of a second major surface of the polymer electrolyte; at leastone of the anode and cathode catalyst layers comprising a first catalystcomposition comprising a noble metal, other than ruthenium; and a secondcomposition comprising a metal oxide, wherein the second composition hasbeen treated with a fluoro-phosphonic acid compound; the methodcomprising the steps of: dissolving the fluoro-phosphonic acid compoundin a solvent to form a fluoro-phosphonic acid compound dispersion;dispersing the fluoro-phosphonic acid compound dispersion with a metaloxide; removing the solvent after dispersing the fluoro-phosphonic acidcompound dispersion with the metal oxide to form the treated secondcomposition; and providing the treated second composition to at leastone of the anode catalyst layer and the cathode catalyst layer.

These and other aspects of the invention are evident upon reference inthe attached drawings and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows the dispersion properties of the untreated RuIrOx powderin water.

FIG. 1b shows the dispersion properties of the treated RuIrOx powder inwater.

FIG. 2 shows the cell reversal tolerance test results of the fuel cellswith the treated and untreated catalyst compositions on the anode.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of the various embodiments ofthe invention. However, one skilled in the art will understand that theinvention may be practiced without these details. In other instances,well-known structures associated with fuel cells, fuel cell stacks,batteries and fuel cell systems have not been shown or described indetail to avoid unnecessarily obscuring descriptions of the embodimentsof the invention.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

A “corrosion resistant support material” is at least as resistant tooxidative corrosion as Shawinigan acetylene black (Chevron ChemicalCompany, TX, USA).

An electrochemical fuel cell includes a polymer electrolyte interposedbetween an anode electrode and a cathode electrode, a cathode catalystlayer between the polymer electrolyte and the cathode electrode, and ananode catalyst layer between the polymer electrolyte and the anodeelectrode. In one embodiment, the anode catalyst layer includes a firstcatalyst composition comprising a noble metal; and a second catalystcomposition comprising a metal oxide; wherein the second catalystcomposition has been treated with a fluoro-phosphonic acid compound. Inspecific embodiments, the metal oxide may be a ruthenium-containingmetal oxide and/or an iridium-containing metal oxide.

The inventors surprisingly discovered that by treating theruthenium-containing metal oxide with a fluoro-phosphonic acid compound,performance degradation due to ruthenium dissolution under voltagecycling conditions (i.e., start-up/shutdown) was reduced, thusindicating the ruthenium dissolution was reduced. Without being bound bytheory, the inventors suspect that covalent bonding of fluoroalkylphosphonic acid with metal oxides through a self-assembly condensationprocess, which increased its hydrophobicity and reduced degradation.

However, the inventors expected that by treating theruthenium-containing metal oxide with a fluoro-phosphonic acid compound,cell reversal tolerance would be reduced because water would be pushedaway from the ruthenium-containing metal oxide (due to the hydrophobicfluorinated alkyl groups), which would reduce the capability of themetal oxide to electrolyze water. It was surprisingly discovered thatcell reversal tolerance was generally unaffected with the treatment.Furthermore, the inventors discovered that cell reversal tolerance wasalso improved for iridium-containing metal oxides. Without being boundby theory, it is suspected that the treatment forms a thin layer offluoro-phosphonic acid at the surface of the ruthenium-containing metaloxide that renders it hydrophobic through the self-assembled surface viacovalent bonding, without significantly affecting the reaction sites (orsurface area) for water electrolysis. In other words, it is suspectedthat the treatment renders a change in the local relative humidity ofthe catalyst layer without negatively affecting cell reversal tolerance.

In some embodiments, the average molecular weight of thefluoro-phosphonic acid compound ranges from about 200 to 1200. Inspecific embodiments, the average molecular weight of thefluoro-phosphonic acid compound ranges from about 300 to 1000.

In some embodiments, the fluoro-phosphonic acid compound has a chainlength of six to twelve carbons in its backbone.

In one embodiment, the fluoro-phosphonic acid compound is afluoroalkyl-phosphonic acid compound.

In specific embodiments, the fluoroalkyl-phosphonic acid compound is aperfluoro-phosphonic acid compound, such as 2-perfluorohexyl ethylphosphonic acid and 1H, 1H, 2h, 2H-heptadecaflurorodec-1-yl phosphoricacid (C10-PFPA).

The first catalyst composition comprises at least one noble metal. Thenoble metal may comprise Pt or an alloy of Pt. In embodiments where a Ptalloy catalyst is employed, the alloy may include another noble metal,such as gold, ruthenium, iridium, osmium, palladium, silver; andcompounds, alloys, solid solutions, and mixtures thereof. In someembodiments, the first catalyst composition comprises a mixture of anoble metal and non-noble metal, such as cobalt, iron, molybdenum,nickel, tantalum, tin, tungsten; and compounds, alloys, solid solutions,and mixtures thereof. While noble metals are described for the firstcatalyst composition, it is expected that non-noble metals, such asthose described above, can also be used as the first catalystcomposition in some applications.

The first catalyst composition may either be unsupported or supported indispersed form on a suitable electrically conducting particulatesupport. In some embodiments, the support used is itself tolerant tovoltage reversal. Thus, it is desirable to consider using supports thatare more corrosion resistant.

The corrosion resistant support material may comprise carbon, ifdesired. High surface area carbons, such as acetylene or furnace blacks,are commonly used as supports for such catalysts. Generally, thecorrosion resistance of a carbon support material is related to itsgraphitic nature: the more graphitic the carbon support, the morecorrosion resistant it is. Graphitized carbon BA (TKK, Tokyo, JP) has asimilar BET surface area to Shawinigan acetylene carbon and is asuitable carbon support material in some embodiments. In otherembodiments suitable carbon support materials may include nitrogen-,boron-, sulfur-, and/or phosphorous-doped carbons, carbon nanofibres,carbon nanotubes, carbon nanohorns, graphenes, and aerogels.

Instead of carbon, carbides or electrically conductive metal oxides maybe considered as a suitable high surface area support for the corrosionresistant support material. For instance, tantalum, titanium and niobiumoxides may serve as a corrosion resistant support material in someembodiments. In this regard, other valve metal oxides might beconsidered as well if they have acceptable electronic conductivity whenacting as catalyst supports.

In embodiments where the first catalyst composition is supported, theloading of the first catalyst composition on the support material isfrom about 20 to about 80% by weight, typically about 20 to about 50% byweight. For a noble metal catalyst, a lower catalyst loading on thesupport is typically preferred in terms of electrochemical surface areaper gram of platinum (ECA), but a higher catalyst loading and coverageof the support appears preferable in terms of reducing corrosion of thesupport and in reducing catalyst loss during fuel cell operation.

In some embodiments, the amount of the first catalyst composition thatis desirably incorporated will depend on such factors as the fuel cellstack construction and operating conditions (for example, current thatmay be expected in reversal), cost, desired lifetime, and so on. Forexample, the catalyst loading of the first catalyst composition mayrange from about 0.05 mg Pt/cm² on the low end for the anode electrodeto about 0.8 mg Pt/cm² on the high end for the cathode electrode. Thesecond composition comprises a metal oxide, wherein the metal oxide maybe, such as, but not limited to, ruthenium oxide, iridium oxide,titanium oxide, cerium oxide, and their mixtures, solid solutions andcomposites thereof. In addition, the metal oxide loading of the secondcomposition may range from about 0.001 mg/cm² to about 0.10 mg/cm². Itis expected that some empirical trials will determine an optimum amountfor a given application. In some embodiments, the metal oxide may besupported on another metal oxide support.

In one embodiment, the metal oxide of the second composition comprisesruthenium. In specific embodiments, the metal oxide is a single-phasesolid solution comprising ruthenium. In further embodiments, the secondcomposition comprises a single-phase solid solution of ruthenium oxide(90:10 mole ratio of Ru:Ir). For example, the metal oxide of the secondcomposition is, but not limited to, RuO₂ and RuIrO₂. As mentioned, themetal oxide may be supported on a catalyst support, such as RuO₂supported on tantalum oxide, titanium oxide or niobium oxide.

In other embodiments, the second catalyst composition comprises a metaloxide comprising iridium, such as a single-phase solid solution ofiridium oxide. As mentioned, the metal oxide may be supported on acatalyst support, such as IrO₂ supported on tantalum oxide, titaniumoxide or niobium oxide.

In further embodiments, a mixture of treated metal oxides may be used,depending on the application.

In other embodiments, the first catalyst composition and the secondcomposition may be in separate, discrete layers in the anode catalystlayer and/or cathode catalyst layer. For example, the first discretelayer with the first catalyst composition is adjacent the membrane andthe second discrete layer with the second composition is adjacent thegas diffusion layer.

In one general method to treat the metal oxide of the secondcomposition, a fluoro-phosphonic acid compound is dissolved in a firstsolvent, a metal oxide is dispersed in another solvent that may be thesame or different from the first solvent, and then the dispersions aredispersed together via conventional methods. The solvents are thenremoved from the dispersion, such as by evaporating the solvent orcentrifuging the dispersion or other methods known in the art, and thenfurther heat-treated at an elevated temperature for an adequate amountof time to form a powder of the fluoro-phosphonic acid-treated metaloxide. The fluoro-phosphonic acid-treated metal oxide may contain about1.0 wt % to about 20.0 wt % of the fluoro-phosphonic acid compound.Without being bound by theory, the phosphonic acid covalently bonds tothe surface oxygen at the surface of the metal oxide with thistreatment.

The second composition may be incorporated in the catalyst layer invarious ways known in the art. In some embodiments, the first catalystand second compositions may be mixed together and the mixture applied ina uniformly distributed common layer or layers on a suitable gasdiffusion layer (GDL), polymer electrolyte membrane, or decal transfersheet. With decal transfer, the catalyst layer may be decal transferredfrom the decal transfer sheet to a GDL to form a gas diffusionelectrode, or may be decal transferred to a polymer electrolyte membraneto form a catalyst-coated membrane. As mentioned previously, the secondcomposition may be supported on the same support material as the firstcatalyst composition, and thus both compositions are already “mixed” forapplication in one or more layers on an anode substrate, cathodesubstrate and membrane.

In further embodiments, the first catalyst composition and the secondcomposition may instead be applied in discrete, separate layers on aGDL, polymer electrolyte membrane, or decal transfer sheet, therebymaking a bilayer or multilayer structure. By applying the first catalystcomposition in one discrete layer and a second composition in a seconddiscrete layer, one may use different ionomers, solvents and processingsteps for each of the first and second compositions.

In another embodiment, the second composition may be non-uniformlydistributed, for example, located where degradation is expected tooccur. Persons of ordinary skill in the art can readily select anappropriate manner of incorporation for a given application.

The anode and cathode catalyst layers typically further include abinder, such as an ionomer and/or hydrophobic agent.

In some embodiments, the through-plane concentration of ionomer in thepresent catalyst layer decreases as a function of distance from thepolymer electrolyte interface. The presence of ionomer in the catalystlayer effectively increases the electrochemically active surface area ofthe catalyst, which requires an ionically conductive pathway to thecathode catalyst to generate electric current.

As previously mentioned, the anode and cathode catalyst layers may beapplied to a GDL to form anode and cathode electrodes, or to a decaltransfer sheet which is then decal transferred to a surface of the GDLor polymer electrolyte, or applied directly to the surface of thepolymer electrolyte to form a catalyst-coated membrane (CCM). Theelectrodes or CCM can then be bonded under heat and/or pressure withother components to form an MEA. Alternatively, the application of thecatalyst layer on the desired substrate may occur at the same time theremaining MEA components are bonded together.

The present catalyst layers may be applied according to known methods.For example, the catalyst may be applied as a catalyst ink or slurry, oras a dry mixture. Catalyst inks may be applied using a variety ofsuitable techniques (e.g., hand and machine methods, including handbrushing, notch bar coating, fluid bearing die coating, wire-wound rodcoating, fluid bearing coating, slot-fed knife coating, three-rollcoating, screen-printing and decal transfer) to the surface of thepolymer electrolyte or GDL. Examples of dry deposition methods includespraying, vacuum deposition and electrostatic powder depositiontechniques.

Catalyst inks typically incorporate the catalysts and binder in asolvent/dispersant to form a solution, dispersion or colloidal mixture.Suitable solvents/dispersants include water, organic solvents such asalcohols and polar aprotic solvents (e.g., N-methylpyrrolidinone,dimethylsulfoxide, and N,N-dimethylacetamide), and mixtures thereof.Depending on the amount of water, one can distinguish water-based inks,wherein water forms the major part of the solvents used, from inkswherein organic solvents form the major part. Catalyst inks may furtherinclude surfactants and/or pore forming agents, if desired. Suitablepore formers include methyl cellulose; sublimating pore-forming agentssuch as durene, camphene, camphor and naphthalene; and pore-formingsolvents that are immiscible with the catalyst ink solvent/dispersant,such as n-butyl acetate in polar aprotic solvent/dispersant systems.

The selection of additional components for the catalyst mixture and thechoice of application method and GDL to which it is applied are notessential to the present invention, and will depend on the physicalcharacteristics of the mixture and the substrate to which it will beapplied, the application method and desired structure of the catalystlayer. Persons of ordinary skill in the art can readily select suitablecatalyst mixtures and application methods for a given application.

EXAMPLES Example 1

Five grams of ruthenium iridium oxide (RuIrO₂) powder purchased fromJohnson Matthey (Reading, UK) was dispersed in 60 mL of ethanol. 0.56 gof 2-perfluorohexyl ethyl phosphonic acid precursor (Unimatech, Japan)was separately dissolved in 5 mL of ethanol. Subsequently, thedispersion containing the RuIrO₂ was probe ultasonicated. The solutioncontaining the dissolved precursor was added to the sonicated dispersionin a dropwise manner (at about a flow rate of 1 mL/min) while stirringat room temperature for about 60 minutes. The dispersion was thentransferred to a petri dish and evaporated at about 60 degrees Celsiusovernight. The free flowing powder was transferred to an oven and heatedto 150 degrees Celsius for 30 minutes, resulting in the final treatedRuIrO₂ powder with about 10 wt % of perfluorohexyl ethyl phosphonicacid.

FIG. 1 shows the picture of treated RuIrO₂ powder (right) in comparisonto non-treated RuIrO₂ (left) dispersed in water. As it can be seen, thetreated RuIrO₂ powder is completely hydrophobic and water could not wetthe surface and, hence, it floats on water, even though the density ofRuIrO₂ is higher than water. On the other hand, non-treated RuIrO₂powder disperses well in water as it is highly hydrophilic.

Example 2

Four grams of ruthenium iridium oxide (RuIrO₂) powder purchased fromJohnson Matthey (Reading, UK) was dispersed in 50 mL of isopropanol. 0.4g of (1H,1H,2H,2H-heptadecafluorodec-1-YL) phosphonic acid (C10-PFPA)precursor (Advanced Technology and Industrial CO., LTD, Hong Kong) wasseparately dissolved in 50 mL of isopropanol. Subsequently, thedispersion containing the RuIrO₂ was probe ultrasonicated. The solutioncontaining the dissolved precursor was added to the sonicated dispersionin a dropwise manner (at about a flow rate of 1 mL/min) while stirringat room temperature overnight. The dispersion was then centrifuged toseparate the catalyst from the solution. The powder at the bottom of thecentrifuge tube was dried in a vacuum oven at 80 degrees Celsius forovernight, resulting in the final treated RuIrO₂ powder with about 2 wt% of C10-PFPA.

Example 3

Five grams of iridium oxide (IrO₂) powder purchased from TanakaKikinzoku International, Inc. (USA) was dispersed in 50 mL of1-propanol. 0.35 g of (1H,1H,2H,2H-heptadecafluorodec-1-YL) phosphonicacid (C10-PFPA) precursor was separately dissolved in 30 mL of1-propanol. Subsequently, the dispersion containing the IrO₂ was probeultrasonicated. The solution containing the dissolved precursor wasadded to the sonicated dispersion in a dropwise manner (at about a flowrate of 1 mL/min) while stirring at room temperature. The dispersion wasthen heated up to 100 degrees Celsius for approximately 3 hours toevaporate all the solvent, resulting in the final treated IrO₂ powderwith about 7 wt % of C10-PFPA.

The treated RuIrO₂ powder of Examples 1 and 2, as well as the IrO₂powder of Example 3, were then dispersed in a Nafion®/alcohol mixturewith a platinum-based catalyst, then applied to a decal transfer filmand subsequently decal transferred via heat and pressure to a half CCM(Nafion® 211 membrane with cathode catalyst on one side) to form a fullCCM. Untreated RuIrO₂ powder and untreated IrO₂ powder were alsoincorporated in a similar fashion to form full CCMs.

The MEAs were made with the following electrode structures as listed inTable 1, with the CCM sandwiched between two AvCarb® GDLs (AvCarbMaterials Solutions, Lowell, Mass.). The GDLs were bonded to the CCM viaheat and pressure. The active area of each of the MEAs was 45 cm².

TABLE 1 Anode and cathode catalyst structures for MEAs MEA Example AnodeCathode Comparative 50% Pt supported on graphitized 50% Pt supported ongraphitized MEA 1 carbon black at a catalyst loading of carbon black at~0.4 mg Pt/cm² ~0.1 mg Pt/cm²; Ionomer (23%): Nafion ® ~0.06 mg/cm²untreated RuIrO₂ (single-phase solid solution (90:10 mole ratio Ru/Ir);Johnson Matthey Plc, Reading, UK); Ionomer (23%): Nafion ® MEA 2 50% Ptsupported on graphitized 50% Pt supported on graphitized (RuIrO₂ ofcarbon black at a catalyst loading of carbon black at ~0.4 mg Pt/cm²Example 1) ~0.1 mg Pt/cm²; Ionomer (23%): Nafion ® ~0.06 mg/cm² RuIrO₂(single-phase solid solution (90:10 mole ratio Ru/Ir); Johnson MattheyPlc, Reading, UK) treated with 2- perfluorohexyl ethyl phosphonic acid;Ionomer (23%): Nafion ® MEA 3 50% Pt supported on graphitized 50% Ptsupported on graphitized (RuIrO₂ of carbon black at a catalyst loadingof carbon black at ~0.4 mg Pt/cm² Example 2) ~0.1 mg Pt/cm²; Ionomer(23%): Nafion ® ~0.06 mg/cm² RuIrO₂ (single-phase solid solution (90:10mole ratio Ru/Ir); Johnson Matthey Plc, Reading, UK) treated with C10PFPA; Ionomer (23%): Nafion ® Comparative 50% Pt supported ongraphitized 50% Pt supported on graphitized MEA 4 carbon black at acatalyst loading of carbon black at ~0.4 mg Pt/cm² ~0.05 mg Pt/cm²;Ionomer (23%): Nafion ® ~0.026 mg/cm² IrO₂ (TKK); Ionomer (20%):Nafion ® MEA 5 50% Pt supported on graphitized 50% Pt supported ongraphitized (IrO₂ of carbon black at a catalyst loading of carbon blackat ~0.4 mg Pt/cm² Example 3) ~0.05 mg Pt/cm²; Ionomer (23%): Nafion ®~0.026 mg/cm² IrO₂ (TKK); treated with C10 PFPA; Ionomer (20%): Nafion ®

The MEAs were then tested in a Ballard Standard Test Cell (STC) testfixture with graphite plates. The fuel cells were first conditionedovernight under the following conditions at 1.3 A/cm²:

TABLE 2 Conditioning parameters Temperature 75° C. (coolant) Inlet DewPoint 75° C. (fuel and oxidant) Fuel 100% hydrogen Oxidant air Reactantinlet pressure 5 psig (fuel and oxidant) Reactant flow 4.5 (fuel), 9.0(oxidant) slpm

Cell Reversal Testing

The fuel cells were conditioned overnight at 1.3 A/cm² at the conditionslisted in Table 2. The fuel supply was then switched to humidifiednitrogen and the cell was supplied with 200 mA/cm² of current through anexternal power supply under current control mode. The cell reversaltolerance time was monitored until the cell voltage reached −2.0 V

FIG. 2 shows that the first three fuel cells exhibited almost exactlythe same behavior with almost the same cell reversal times (85 minutesfor MEA 1 and

MEA 1, and 89 minutes for MEA 3). Therefore, the treatment of RuIrO₂with either perfluorohexyl ethyl phosphonic acid or C10-PFPA did notsignificantly impact cell reversal tolerance, which, as discussed above,is a surprising and unexpected result due to the expected lower surfacearea of the RuIrO₂ after treatment.

For iridium oxide, it was shown that MEA 5 with a C10-PFPA-treatediridium oxide had a significantly improved cell reversal time incomparison to Comparative MEA 4, which did not have the C10-PFPAtreatment. Comparative MEA 4 had a cell reversal time of less than 300minutes while MEA 5 had a cell reversal time of greater than 450minutes. Thus, the treatment of IrO₂ with C10-PFPA had a positive effecton cell reversal tolerance.

Anode Accelerated Stress Test

Anode accelerated stress tests (ASTs) were used to simulate potentialspikes that occur during fuel cell start-ups and shutdowns to induceruthenium dissolution and crossover.

The fuel cells were operated at 75° C., 5 psig (136 kPa) pressure, and100% inlet RHs, 70% H₂/30% N₂ for the fuel, and a beginning of life(BOL) polarization was obtained. During the AST, the anode potentialcycled between ˜0 and 0.9 V by switching the fuel between 70% H₂/30% N₂for 1 minute and 100% N2 for 30 seconds, while the cathode potential waskept below 1.0 V to minimize cathode degradation. Unless otherwisestated, all half-cell potentials reported here are relative to thedynamic hydrogen reference electrode (DHE). After the AST, the fuelcells were again operated at the same conditions as the BOLpolarization, and an end of life (EOL) polarization was obtained.

At 1 A/cm², it was shown that MEA 2 had a performance loss of 38 mV lessthan the baseline MEA 1, and MEA 3 had a performance loss of 67 mV lessthan the baseline MEA 1. Therefore, by treating the RuIrO₂ catalyst witheither perfluorohexyl ethyl phosphonic acid or C10-PFPA, the MEAs withthe treated RuIrO₂ catalyst exhibited significantly less performanceloss at 1 A/cm² than the untreated MEA.

While the treated metal oxides have been described for the anodeelectrode in the preceding description, it is contemplated that suchtreated metal oxides may, additionally or alternatively, be used on thecathode electrode. Without being bound by theory, such treated metaloxides are beneficial for improved durability by mitigating carboncorrosion at high cathode potentials by acting as a water electrolysiscatalyst.

While the present electrodes have been described for use in PEM fuelcells, it is anticipated that they may be useful in other fuel cellshaving an operating temperature below about 250° C. They areparticularly suited for acid electrolyte fuel cells, includingphosphoric acid, PEM and liquid feed fuel cells. It is also contemplatedthat this treatment may also be useful for other metal oxides comprisingruthenium. All of the above U.S. patents, U.S. patent applicationpublications, U.S.

patent applications, foreign patents, foreign patent applications andnon-patent publications referred to in this specification and/or listedin the Application Data Sheet, are incorporated herein by reference intheir entirety.

While particular elements, embodiments and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art, particularly in light of theforegoing teachings. It is therefore contemplated by the appended claimsto cover such modifications that incorporate those features comingwithin the scope of the invention.

This application also claims the benefit of U.S. Provisional PatentApplication No. 62/370,144, filed Aug. 2, 2016, and is incorporatedherein by reference in its entirety.

1. A membrane electrode assembly, comprising: a polymer electrolyteinterposed between an anode electrode and a cathode electrode, the anodeelectrode comprising an anode catalyst layer adjacent at least a portionof a first surface of the polymer electrolyte, the cathode electrodecomprising a cathode catalyst layer adjacent at least a portion of asecond surface of the polymer electrolyte, at least one of the anode andcathode catalyst layers comprising: a first catalyst compositioncomprising a noble metal; and a second composition comprising a metaloxide, the second composition has been treated with a fluoro-phosphonicacid compound.
 2. The membrane electrode assembly of claim 1, whereinthe noble metal of the first catalyst composition comprises platinum ora platinum alloy.
 3. The membrane electrode assembly of claim 1, whereinthe noble metal of the first catalyst composition is selected from thegroup consisting of platinum, gold, ruthenium, iridium, osmium,palladium, silver; and compounds, alloys, solid solutions, and mixturesthereof.
 4. The membrane electrode assembly of claim 1, wherein themetal oxide of the second composition is a single-phase solid solutioncomprising ruthenium.
 5. The membrane electrode assembly of claim 1,wherein the metal oxide of the second composition is a single-phasesolid solution comprising iridium.
 6. The membrane electrode assembly ofclaim 1, wherein the metal oxide of the second composition is selectedfrom the group consisting of ruthenium oxide, iridium oxide, titaniumoxide, cerium oxide, and their mixtures, solid solutions and compositesthereof.
 7. The membrane electrode assembly of claim 1, wherein themetal oxide of the second composition is supported on a supportmaterial.
 8. The membrane electrode assembly of claim 1, wherein thefluoro-phosphonic acid compound is a fluoroalkyl-phosphonic acidcompound.
 9. The membrane electrode assembly of claim 8, wherein thefluoro-phosphonic acid compound is 2-perfluorohexyl ethyl phosphonicacid.
 10. The membrane electrode assembly of claim 8, wherein thefluoro-phosphonic acid compound is (1H,1H,2H,2H-heptadecafluorodec-1-yl)phosphonic acid.
 11. The membrane electrode assembly of claim 1, whereinthe second composition comprises about 1.0 wt % to about 20.0 wt % ofthe fluoro-phosphonic acid compound.
 12. The membrane electrode assemblyof claim 1, wherein the fluoro-phosphonic acid compound has a molecularweight of at least
 200. 13. The membrane electrode assembly of claim 1,wherein the first catalyst composition is in a first discrete layer andthe second composition is in a second discrete layer in the at least oneof the anode and cathode catalyst layers. 14.-24. (canceled)